1. Field of the Invention
This invention relates to a thin film type optical device, i.e., an optical device having a thin film light waveguide path, and more particularly to a thin film type optical device used as a light deflecting device, a light modulator, a spectrum analyzer, a light correlator or the like utilizing the electro-optical (EO) effect or the acousto-optical (AO) effect.
2. Description of the Prior Art
When forming a thin film type optical device as described above, lithium niobate (hereinafter referred to as LiNbO.sub.3) crystal and lithium tantalate (hereinafter referred to as LiTaO.sub.3) crystal, both excellent in piezo-electric property, EO effect and AO effect and having a small light propagation loss, have heretofore been widely used as the substrate thereof. As a typical method of making a thin film light waveguide path by the use of such a crystal substrate, there is a method of heat-diffusing titanium (hereinafter referred to as Ti) on the surface of said crystal substrate at a high temperature, thereby forming on the surface of said crystal substrate a light waveguide path layer having a refractive index slightly greater than the refractive index of the substrate. However, the thin film light waveguide path made by this method has suffered from the disadvantage that it is susceptible to optical damage and only light of very small power can be introduced into the waveguide path. The optical damage refers to "a phenomenon that when the intensity of light input to the light waveguide path has been increased, the intensity of the light propagated through the light waveguide path and taken out therefrom is not increased in proportion to the intensity of said input light by scattering."
As a method for making a light waveguide path to overcome said optical damage, there is a method of heat-treating a LiNbO.sub.3 or LiTaO.sub.3 crystal substrate at a high temperature, externally diffusing lithium oxide (hereinafter referred to as Li.sub.2 O) from within said crystal substrate and forming near the surface of the substrate a lithium (hereinafter referred to as Li) empty grating layer having a refractive index slightly greater than that of the substrate. It is shown in the literature [R. L. Holman & P. J. Cressman: IOC, 90, 28 April (1981)] that by said Li.sub.2 O external diffusing method, the threshold value of optical damage becomes higher than by the Ti internal diffusing method.
Now, when it is desired to realize a light deflecting device or a light modulator by the utilization of the acousto-optical effect or the electro-optical effect, it is important in the formation of the element to enhance the efficiency of each said effect. As a typical example utilizing the acoustooptical effect, there is a method of applying a high frequency electric field to a comb type electrode formed on a light waveguide path by photolithography and exciting an elastic surface wave on the light waveguide path. It is known that in this case, the interaction of the elastic surface wave excited on the light waveguide path and the waveguide light propagated through the light waveguide path increases as the energy distribution of the waveguide light is confined near the surface of the substrate [C. S. Tsai, IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS, Vol. CAS-26, 12, 1979].
From the viewpoint of utilizing said interaction to its maximum, the thickness of the light waveguide path layer (Li empty grating layer) formed by the aforedescribed Li.sub.2 O external diffusing method must be considerably great, say, of the order of 10-100 .mu.m, because the variation in the refractive index thereof is small. This is not preferable in that the energy distribution of the waveguide light spreads in the direction of thickness. Accordingly, where the thin film type optical device made by the aforedescribed Li.sub.2 O external diffusing method is utilized in said light deflecting device or the like, it has been difficult to realize a device of high efficiency.
On the other hand, the ion exchange method is known as another method for making a thin film light waveguide path which overcomes optical damage. According to this method, a LiNbO.sub.3 or LiTaO.sub.3 crystal substrate is heat-treated at a low tempeature in a molten salt such as thallium nitrate (hereinafter referred to as TlNO.sub.3), silver nitrate (hereinafter referred to as AgNO.sub.3) or kalium nitrate (hereinafter referred to as KNO.sub.3) or in a weak acid such as benzoic acid (C.sub.6 H.sub.5 COOH), whereby the lithium ions (Li.sup.+) in the crystal substrate are exchanged with the ion species such as protons (H.sup.+) in the weak acid and a light waveguide path layer having a great refractive index difference (.DELTA.n-0.12) is formed.
The threshold value of optical damage of the thin film light waveguide path made by the above-described ion exchange method has good characteristics improved several tens of times as compared with that of the thin film light waveguide path made by the Ti diffusion. On the other hand, it has suffered from the problem that the piezo-electric property and the electro-optical characteristic inherent to LiNbO.sub.3 or LiTaO.sub.3 crystal are reduced by said ion exchange process and where it is used, for example, in a light deflecting device, the diffraction efficiency of the waveguide light is reduced.